A wide variety of implantable medical systems that deliver a therapy or monitor a physiologic condition of a patient have been clinically implanted or proposed for clinical implantation in patients. The implantable medical system may include an implantable medical lead connected to an implantable medical device (IMD). For example, implantable leads are commonly connected to implantable pacemakers, defibrillators, cardioverters, or the like, to form an implantable cardiac system that provides electrical stimulation to the heart or sensing of electrical activity of the heart. The electrical stimulation pulses can be delivered to the heart and the sensed electrical signals can be sensed by electrodes disposed on the leads, e.g., typically near distal ends of the leads. Implantable leads are also used in neurological devices, muscular stimulation therapy, gastric system stimulators and other implantable medical devices (IMDs).
Patients that have implantable medical systems may benefit, or even require, various medical imaging procedures to obtain images of internal structures of the patient. One common medical imaging procedure is magnetic resonance imaging (MRI). MRI procedures may generate higher resolution and/or better contrast images (particularly of soft tissues) than other medical imaging techniques. MRI procedures also generate these images without delivering ionizing radiation to the body of the patient, and, as a result, MRI procedures may be repeated without exposing the patient to such radiation.
During an MRI procedure, the patient or a particular part of the patient's body is positioned within an MRI device. The MRI device generates a variety of magnetic and electromagnetic fields to obtain the images of the patient, including a static magnetic field, gradient magnetic fields, and radio frequency (RF) fields. The static magnetic field may be generated by a primary magnet within the MRI device and may be present prior to initiation of the MRI procedure. The gradient magnetic fields may be generated by electromagnets of the MRI device and may be present during the MRI procedure. The RF fields may be generated by transmitting/receiving coils of the MRI device and may be present during the MRI procedure. If the patient undergoing the MRI procedure has an implantable medical system, the various fields produced by the MRI device may have an effect on the operation of the medical leads and/or the IMD to which the leads are coupled. For example, the gradient magnetic fields or the RF fields generated during the MRI procedure may induce energy on the implantable leads (e.g., in the form of an electrical current), which may cause oversensing by the IMD. In other words, the IMD may incorrectly detect a cardiac signal when one is not present. Oversensing may result in the IMD delivering therapy when it is not desired or withholding therapy when it is desired.
A number of techniques have been described for reducing the effects of oversensing during an MRI procedure. For example, U.S. Pat. No. 7,693,568 to Zeijlemaker (referred to herein as “the '568 patent”) describes a signal processing algorithm for discounting MRI artifacts. As described with reference to FIG. 5B of the '568 patent, when electrical activity and gradient field activity are sensed, per step 402, a decision is made at step 403 based upon the timing of the sensed electrical events. If electrical events, for example sensed by electrodes of a lead, do not coincide with gradient field activity whether or not the electrical events coincide with extrapolated cardiac events, the events are counted as an actual cardiac event at step 404C. If sensed electrical events do coincide with the sensed gradient fields but not with extrapolated cardiac events, the events are counted as noise at step 404B. If sensed electrical events coincide with both of the extrapolated cardiac events and the sensed gradient fields, the events are counted as “virtual” or potential cardiac events at step 404A. The “virtual” events are processed by the device, according to typical state of the art limitations for such devices, for control of therapy delivery to maintain physiological cardiac function. Per step 405, when a consecutive count of noise events and “virtual” cardiac events exceeds a predetermined number, electrical sensing is ignored at step 406. If electrical sensing is ignored, then the device may switch into a prescribed mode of therapy delivery, for example pacing stimulation at a prescribed number of beats per minute.
As another example, U.S. Pat. No. 5,697,958 to Paul et al. (referred to herein as “the '958 patent”) describes a demand pacemaker that responds to a message that electromagnetic interference (EMI) has been detected in the same manner that it would respond if it sensed that the heart of the patient has failed to perform an expected event, that is, by producing a pacing signal to pump the chamber in which heart signals are being sensed. With reference to FIG. 7 of the '958 patent, upon the start 210 of the illustrated process 200 the microprocessor initiates an escape interval at 212, that is, a time interval during which the pacemaker waits for the heart to perform an event, such as producing a cardiac electrical signal, for the pacemaker to sense and during which the pacemaker will not send a pacing signal to the heart. At 214 the pacemaker waits for the escape interval to expire, or the pacemaker to sense an event at the heart, whichever occurs first. The microprocessor inquires at 216 whether a heart event was sensed by the expiration of the escape interval.
If the answer is affirmative, the microprocessor of the '958 patent proceeds from 216 to determine at 218 whether it has received an “EMI present” flag, or an EMI coincidence flag, for example, from the noise detector of the present invention during the time period since the start of the escape interval at 212. If EMI is not present, a conclusion is reached that the pacemaker is functioning properly and the sensed signal is a true indication of the occurrence of the heart event and steps 220-224 are performed. However, if EMI is determined at 218 to be present, the microprocessor cannot know whether the signal sensed by the pacemaker was actually the result of a heart event or the result of EMI in the pacemaker electronics. Therefore, if EMI is determined at 218 to be present, the pacemaker proceeds as if the patient needed assistance. The pacemaker waits at 228 for the current escape time interval to be completed and generates a stimulus pulse at 226.